docs: def xmlrpc_feedback(self, app_id):
      """ Queries the Apple APNS feedback server for inactive app tokens. Returns
      a list of tuples as (datetime_went_dark, token_str).
      
        Arguments:
            app_id   the app_id to query
        Returns:
            Feedback tuples like (datetime_expired, token_str)
      """
      
      return self.apns_service(app_id).read().addCallback(
        lambda r: decode_feedback(r))